Processes to produce single-walled carbon nanotubes (SWCNT) typically produce a mixture of semiconducting single-walled carbon nanotubes (sc-SWCNT) and metallic single-walled carbon nanotubes (m-SWCNT). sc-SWCNT typically forms about 75 wt % of the mixture in as-produced SWCNT. For application to semiconductor devices, SWCNT of higher purity is desired, as the fully conductive m-SWCNT may interfere with proper functioning of semiconductor devices.
Significant progress in the purification of sc-SWCNT in recent years makes this type of material highly promising for printable electronic applications. Thin film transistors (TFT) with mobilities higher than 30 and on/off ratios above 106 can now be readily achieved (Ding 2014; Ding 2015). However, to make printed circuits with integrated TFT devices remains a challenge. The major issue for this challenge is related to a lack of proper sc-SWCNT inks for industrial printing, especially for roll-to roll printing. Current purification techniques only provide sc-SWCNT dispersions in either aqueous or non-polar organic solvents. sc-SWCNT dispersions in aqueous solution are stabilized by large amounts of surfactant, usually with a weight ratio of 1000/1 of surfactant to nanotubes. The large amount of surfactant prevents their formulation into relevant inks for common printing techniques such as gravure and flexo. Though the conjugated polymer used to stabilize the nanotube dispersion in a non-polar organic solvent such as toluene is at a low weight ratio (usually 1/1 to about 5/1), and does not show significant deleterious effects on device performance (mobility; on/off), this type of dispersion is also difficult to formulate due to a lack of dispersability in common solvents used in commercial printing, where polar organic solvents with a higher viscosity, such as alkyl carbitol, are preferred.
There is little present knowledge related to making stable hydrophilic dispersions of enriched conjugated polymer wrapped sc-SWCNT materials. Ligand exchange has been demonstrated (Stranks 2013a; Stranks 2013b) but not with the intent to dramatically change the dispersability from hydrophobic to hydrophilic systems, nor for the purpose of making relevant ink systems for use in commercial environments. There is no literature of which we are aware, which describes the enrichment of sc-SWCNT in polar organic solvents, such as alkyl carbitols, and which provides an efficient path towards the desired products, obviating the need for ligand exchange should that be preferred. Present literature (Wang 2015) indicates that sorting SWCNTs in a polar solvent like tetrahydrofuran (THF) does not yield selective dispersions of sc-SWCNT.
Polymers with oligoether side groups, such as poly(9,9-di(methoxyethoxyethoxy ethyl)fluorene) have been proposed to form composite materials from as-prepared carbon nanotubes for application as light-emitting materials (Liu 2011). However, such materials have not been reported for the preparation of composite materials with highly purified semi-conducting single-walled carbon nanotubes (sc-SWCNT). Because purified/enriched sc-SWCNT are usually wrapped by surfactants or conjugated polymers based on the technique used for the nanotube purification, composite materials of sc-SWCNT/polymer with oligoether side groups cannot be prepared by simply mixing sc-SWCNT with the corresponding polymer as proposed in Liu 2011.
A method for ligand exchange in which a polyfluorene polymer (PF) is displaced by polythiophene with alkyl side chains has been proposed (Stranks 2013a; Stranks 2013b). However, the prior art does not teach how to effectively take a hydrophobic composite and render the composite hydrophilic. The prior art also does not teach how to successfully perform an enrichment process that yields high purity sc-SWCNT using a hydrophilic conjugated polymer/solvent system.
There remains a need to be able to efficiently disperse enriched sc-SWCNT in polar organic solvents to form stable printable inks.